The present invention relates to lithium transition metal oxide based powders for use in a rechargeable battery, that are coated with a lithium absorbing glassy coating. In particular high nickel containing powders are used, in order to improve their high temperature stability.
Previously LiCoO2 was the most used cathode material for rechargeable lithium batteries. However, a substitution of LiCoO2 by lithium nickel oxide based cathodes and by lithium nickel manganese cobalt oxides has recently started. In these substitute materials, depending on the choice of metal composition, different limitations occur or challenges still need to be solved. For simplicity reasons, the term “Lithium nickel oxide based cathodes” will be further referred to as “LNO”, and “lithium nickel manganese cobalt oxides” will be further referred to as “LNMCO”.
One example of an LNO material is LiNi0.80Co0.15Al0.05O2. There is also a special class of LNO material described in WO2010-094394. It concerns a material with general formula LiaNixCoyMzO2±eAf, with 0.9<a<1.1, 0.3≤x≤0.9, 0≤y≤0.4, 0<z≤0.35, e<0.02, 0≤f≤0.05 and 0.9<(x+y+z+f)<1.1; M consisting of either one or more elements from the group Al, Mg, and Ti; A consisting of either one or both of S and C, and wherein the material composition—i.e. its Ni and M content—is dependent on the particle size. LNO has a high capacity, however it is difficult to prepare, since typically a carbon dioxide free atmosphere (e.g. a pure oxygen atmosphere) is needed and special carbonate free precursors—like lithium hydroxide—are used, instead of lithium carbonate. Hence such manufacturing restraints tend to increase the cost of this material considerably.
LNO is also a very sensitive cathode material. It is not fully stable in air, which makes large scale battery production more difficult, and in real batteries—due to its low thermodynamic stability—it is responsible for a poor safety record. Finally, it is very difficult to produce lithium nickel oxide with a low content of soluble base: it is known that lithium located near to the surface is thermodynamically less stable and can go into solution, but lithium in the bulk is thermodynamically stable and cannot go to dissolution. Thus a gradient of Li stability exists, between lower stability at the surface and higher stability in the bulk. By determining the “soluble base” content by pH titration, based on the ion exchange reaction (LiMO2+δ H+←→Li1−δHδMO2+δ Li+, M being one or more transition metals), the Li gradient can be established. The extent of this reaction is a surface property. The soluble base can be of the LiOH or the Li2CO3 type, as is described in co-pending application EP11000945.3.
In US2009/0226810A1 the problem of soluble base is further discussed. The ‘soluble base’ problem is severe because a high base content is often connected with problems during battery manufacturing: during slurry making and coating high base causes a degradation of the slurry (slurry instability, gelation) and high base is also a responsible for poor high temperature properties, like excessive gas generation (swelling of the batteries) during high temperature exposure. In the case of a flexible casing, for example in all designs like prismatic or pouch type—with the exception of cylindrical cells—the cell bulges which is a failure of the battery.
An example of LNMCO is the well known Li1+xM1−xO2 with M=Mn1/3Ni1/3Co1/3O2, where the manganese and nickel content is about the same. “LNMCO” cathodes are very robust, easy to prepare, have a relatively low content of cobalt and thus generally tend to cost less. Their main drawback is a relatively low reversible capacity. Typically, between 4.3 and 3.0V the capacity is less than or about 160 mAh/g, compared with 180-190 mAh/g for LNO cathodes. A further drawback of LNMCO compared with LNO is the relatively low crystallographic density—so the volumetric capacity is also less—and a relatively low electronic conductivity.
In between LNO and LNMCO type materials we can situate “Nickel rich lithium nickel manganese cobalt oxides” Li1+x′M1−x′O2 where M=Ni1−x−yMnxCoy or M=Ni1−x−y−zMnxCoyAlz, with Ni:Mn larger than 1, i.e. typically values for Ni:Mn of 1.5 to 3, and a Co content “y” typically between 0.1 and 0.3 (0.1≤y≤0.3), and 0≤z≤0.05. For simplicity we refer to this class of materials as “LNMO”. Examples are M=Ni0.5Mn0.3Co0.2, M=Ni0.67Mn0.22Co0.11, and M=Ni0.6Mn0.2Co0.2. A special class of LNMO material is described in WO2009/021651. It concerns Li1+aM1−aO2±bM′kSm with −0.03<a<0.06, b≅0 (or b<0.02), M being a transition metal composition, with at least 95% of M consisting of either one or more elements of the group Ni, Mn, Co and Ti; Mn, Co and Ti; M′ being present on the surface of the powderous oxide, and M′ consisting of either one or more elements of the group Ca, Sr, Y, La, Ce and Zr, with 0.0250<k≤0.1 in wt %; and 0.15<m≤0.6, m being expressed in mol %.
Compared with LNO, LNMO can be prepared by standard processes (using a Li2CO3 precursor) and no special gas—such as oxygen—is needed. Compared to LNMCO, LNMO has a much higher intrinsic capacity and possibly a lower tendency to react with electrolyte (which is normally characterized by dissolution of Mn) at elevated temperature. Thus it becomes apparent that LNMO plays a major role in the substitution of LiCoO2. Generally, the base content increases, and the safety performance tends to deteriorate with increasing Ni:Mn ratio.
In LNO most of the Ni is divalent. In LNMO some nickel is divalent and some nickel is trivalent. Generally there exist a tendency that with increasing Ni(3+)
(1) reversible capacity (at given voltage range) increases,
(2) it becomes more difficult to prepare high quality product,
(3) the product becomes more sensitive (for moisture, air exposure etc.), and
(4) the content of soluble base increases.
Generally LNO has a very high base content and LNMCO a relatively low content. LNMO has less base than LNO but more than LNMCO. It is widely accepted that high Mn content helps to improve safety.
A high base content is related to moisture sensitivity. In this regard LNMO is less moisture sensitive than LNO but more sensitive than LNMCO. Directly after preparation, a well prepared LNMO sample has a relatively low content of surface base, and if it is well prepared most of the surface base is not Li2CO3 type base. However, in the presence of moisture, airborn CO2 or organic radicals reacts with LiOH type base to form Li2CO3 type base. Similar, the consumed LiOH is slowly re-created by Li from the bulk, thus increasing the total base (total base=mol of Li2CO3+LiOH type base). At the same time, the moisture (ppm H2O) increases. These processes are very bad for battery making. Li2CO3 and moisture are known to cause severe swelling, and to deteriorate the slurry stability. Hence it is desired to decrease the moisture sensitivity of LNMO and LNO materials.
Thermal stability (safety) is related to interfacial stability between electrolyte and cathode material. A typical approach to improve the surface stability is by coating. Many different examples of conventional coatings are available in literature in general and in patent literature in particular. There are different ways to categorize coatings. For example, we can distinguish between ex-situ and in-situ coating. In ex-situ coating a layer is coated onto the particles. The coating can be obtained by dry or wet coating. Generally the coating is applied in a separate process involving at least the coating step and generally an additional heating step. Thus the total cost of the process is high. Alternatively, in some cases an in-situ coating—or self organized coating—is possible. In this case the coating material is added to the precursor blend before cooking, and during cooking separate phases form, preferable the coating phase becomes liquid, and if the wetting between LiMO2 and the coating phase is strong then a thin and dense coating phase ultimately covers the electrochemical active LiMO2 phase. Evidently, in-situ coating is only efficient if the coating phase wets the core.
We can also distinguish between cationic and anionic coating. An example of cationic coating is Al2O3 coating. Examples for anionic coating are fluoride, phosphate, silicate coating and the like. In US2010/0190058 lithium metal oxide particles are provided with a coating of lithium-metal-polyanionic, lithium-metal-phosphate or lithium-metal-silicate compounds. The coating compounds are fully lithiated and are not able to bind lithium situated at the surface of the metal oxide particles.
We can further distinguish between inorganic and organic coatings. An example of an organic coating is a polymer coating. One advantage of polymer coating is the possibility of obtaining an elastic coating. On the other hand, problems arise from poor electronic conductivity, and sometimes the poor transport of lithium across the polymer. Generally, a polymer coating more or less adheres to the surface, but it does not chemically change the surface.
There cannot be found any experimental data in the prior art that would show that the above described approaches are effective to improve the cited limitations of LNO and LNMO materials. The present invention discloses a new unified approach to deal with all of the above mentioned shortcomings, with focus on lower content of soluble base but also addressing thermal stability and moisture sensitivity.
To summarize:    (1) LNMCO is a robust material but has severe capacity limitations,    (2) LNO has very high capacity but is very sensitive and requires expensive preparation route. Its stability needs to improve, and a lower content of soluble base is preferred,    (3) LNMO can be prepared by a cheap route. It has high capacity but stability needs to improve. Also a lower content of soluble base is preferred.
The present invention aims to improve the stability of LNO and LNMO materials, and to provide LNMO as a high capacity alternative for LNMCO materials.